It probably [ could have ] cooled down enough , during the plug change to get back to intermittant , please keep in mind this is purely speculation , but based on the process of elimination described in your post above of all the thing that were tried , I believe you still have a condition. As always , I hope the answer is the least expensive thing in the list.And if it becomes a scenario where the problem actually is just a bad ground wire , then woo hoo ! By the way , have you gone through and cleaned and checked all the grounds? Spark demands go up in ignitions with , hi - comp. pistons , extreme rpm ceiling increases , super heavy loads seen at the crank. These are just a few items that cause the spark plug to pull / demand more sprk energy. When the demand goes up , a bad ground has been known to cause this type of problem. crap - im rambling again , sorry.